locate the Nginx configuration directory /etc/nginx/conf.d/ (or unlikely /opt/nginx/conf/), or the main configuration file (etc/nginx/nginx.conf) and look for the server block that creates the logs. Add these lines:
server {
[...]
access_log off;
error_log off;
[...]
}
This will disable logging for the given server block.
Disclaimer: This is a guessed answer. I never worked with the Graylog OVA.